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Author: Tyler B Cote
Requested Type: Poster
Submitted: 2018-02-28 12:49:25

Co-authors: C. C. Hegna, M. Willensdorfer, E. Strumberger, W. Suttrop, H. Zohm, ASDEX Upgrade Team

Contact Info:
University of Wisconsin-Madison
6237 Pheasant Lane, Apt. 41
Middleton, WI   53562
United States

Abstract Text:
Application of 3D magnetic perturbations has been shown to destabilize ideal MHD ballooning modes in tokamak pedestals [1]. Recent observations on ASDEX-Upgrade have shown toroidally localized MHD activity in the presence of strong applied 3D magnetic perturbations [2]. In this work, we utilize infinite-n ballooning analysis to study MHD stability in 3D VMEC equilibria modeling the AUX experiment, in hopes to better understand the underlying destabilization mechanisms behind the localization of the ballooning modes.

VMEC equilibria with varied 3D magnetic perturbation strength are used to determine the relation between 3D edge displacement and ballooning localization. It is found that a minimum amount of 3D shaping is necessary for the observation of ballooning instabilities. Analysis shows localization of the ballooning mode to specific field-lines corresponding to minima in the local magnetic shear. 3D distortion of the flux surfaces cause significant change in the normal torsion, a key component of the local shear, and act as the primary mechanism for ballooning destabilization on certain field-lines. The minimum critical 3D distortion appears tied to activating the 3D response in the normal torsion. Our analysis agrees well with the experimentally observed localization of the ballooning modes in regions of small local magnetic shear.

[1] T.M. Bird and C.C. Hegna, Nucl. Fusion, 53 (2013)

[2] M. Willlensdorfer et al., Physics Review Letters, 119 (2017).